The reaction catalyzed by creatine kinase (CK), the transfer of the phosphoryl group between phosphocreatine (PCr) and ATP, functions in vivo to maintain constant [ATP] independently of differences in ATP turnover caused by variations in workload. Thus, this energy reserve system is used to support the contractile reserve of the heart. Observations made in our laboratory suggest that decreased energy reserve via the CK system (decreased Vmax and [creatine], increased BCK and decreased MCK and mitochondrial CK isoenzymes) may contribute to depressed ventricular function in the failing heart, supporting the hypothesis that decreased energy reserve via the CK system impairs contractile reserve in the failing myocardium. Combining the tools of physiology, biophysics, biochemistry and molecular biology, we propose a set of experiments to test this hypothesis. A major goal of this research proposal is to determine the relations among CK reaction velocity measured in vivo, baseline systolic and diastolic performance, and contractile reserve in chronically failing intact mammalian hearts. These experiments will define these relations with all perturbations present (changes in V max [substrate] and isoenzyme distribution) but many other protein systems in the failing cell will also be different. To test for cause-and-effect relations, it will be necessary to change each parameter independently. We propose to do this using transgenic technology. Our goal is to create three mice whose cardiac CK system mimics one, and only one, of the perturbations that we now know characterize the failing myocardium; overexpressed BCK, overexpressed mitochondrial CK or no creatine transporter. For each, we will apply the biochemical, biophysical and physiological tools that we have developed to define the whole organ enzymology of the CK system to the intact isolated working heart. In this way, the physiological significance of specific changes in the CK system will be defined. We will also define the whole organ enzymology of specific isoenzymes.